Thermochronological analysis of the dynamics of the Southern Alps, New Zealand

The isotopic ages of material exposed close to the Alpine fault in the Southern Alps, New Zealand, vary in a consistent fashion along the orogen. For the K-Ar system, muscovite and biotite ages display two distinct spatial trends that meet in the area of Fox Glacier, where a strong relationship is observed between K-Ar age and altitude. Excess argon is present in samples between Mount Kinnaird and Haast Pass in the southwest of the Southern Alps, giving unrealistically old, inconsistent K-Ar ages across this region. Incorporation of excess argon appears to be highly variable, however, and minimization of its influence on dynamical interpretations of thermochronology is possible through considering only the youngest age from a given area. Such treatment indicates that excess argon contamination does not play an important role in the systematic K-Ar age variation identified along the Southern Alps. The observed age distribution is consistent with estimates of the modern rapid uplift and exhumation of the Southern Alps beginning ca. 5 Ma. K-Ar mica ages older than this south of the Copland Valley indicate that substantial late Cenozoic exhumation occurred along the Southern Alps prior to the development of the modern tectonic regime, but at rates lower than at present. Total exhumation during the current phase of tectonism in the Southern Alps decreases from the Whataroa River southward. North of Fox Glacier, the Southern Alps have attained a dynamical steady state, such that the observed distribution of exhumation will not change significantly with further deformation and denudation under current conditions.